The object of screening suspensions of fibrous materials is to separate coarse particles in the form of shives, fibrous bundles and other impurities from the fibrous material itself, or to fractionate same, i.e., to separate long and short fibers from each other. The screening normally is carried out in screening devices comprising a stationary or rotary screening cylinder. The screening cylinder is provided with apertures or slits, through which the fibers, but not the impurities, can pass.
Screening cylinders of this type are normally manufactured by drilling apertures or milling slits in a metal sheet, which is then formed into a cylinder. In order to obtain an open area as large as possible, the apertures or slits must be arranged rather densely.
In another method of forming such screening cylinders, the cylinder is made of bars, which are held together by supporting rings in a manner such that gaps are formed between the bars, and these gaps extend along the entire length of the cylinder. Such a screening cylinder has many advantages over a conventionally slitted screening cylinder. For example, they possess a greater open area and a higher selectivity with respect to shive separation and fractionating. Alternatively, the bars can be formed of a metal sheet by arranging a great number of parallel grooves therein.
It can also be advantageous to form the surface of the screening cylinder in such a manner that the upstream edges of the gaps are located on a higher level than the downstream edges, as seen in the direction of the relative motion of the suspension. This offers many advantages from the point of view of a screening. Screening cylinders comprising gaps of this type have, for example, a higher capacity and a high capability of Shives separation. In certain cases, however, such as when capacity is the main priority, it can be desirable to form a planar screen surface, or in a manner such that the downstream edges are located higher than the upstream edges.
By forming the screening cylinder according to the present invention, it is now possible to additionally improve the efficiency and runnability of the screening device.